1. Field of the Invention
The present invention is related to a method of transmitting RRC messages, and more particularly, to a method of transmitting RRC messages in the RRC-idle mode.
2. Description of the Prior Art
The 3rd Generation Partnership Project (3GPP) has developed a universal mobile telecommunications system (UMTS) which adopts a wideband code division multiple access (WCDMA) technology capable of providing high frequency spectrum utilization, universal coverage, and high-speed/quality multimedia data transmission. In the UMTS, a radio access network known as a universal terrestrial radio access network (UTRAN) includes multiple base stations, commonly known as Node-Bs (NBs), for communicating with multiple user equipment (UE). Furthermore, a long-term evolution (LTE) system is now being developed by the 3GPP in order to further improve performance of the UMTS to satisfy users' increasing needs. The LTE system includes a new radio interface and radio network architecture which provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved NBs (eNBs) for communicating with multiple UEs. LTE radio interface protocols include an access stratum (AS) layer and a non-access stratum (NAS) layer. The AS layer includes a physical layer (L1), a data link layer (L2) and a network layer (L3). The network layer controls a radio resource control (RRC layer). The data link layer includes a packet data convergence protocol (PDCP), a radio link control (RLC) and a medium access control (MAC). The NAS layer handles the communication between the UE and the core network (CN) which includes a mobility management entity (MME), a serving gateway and other devices.
Multimedia Broadcast Multicast Service (MBMS) is a Point-to-Multipoint (PTM) interface specification designed to provide efficient delivery of broadcast and multicast services within 3GPP cellular networks. Examples of MBMS interface specifications include those described in UMTS and LTE communication specifications. For broadcast transmission across multiple cells, the specifications define transmission over single-frequency network configurations. Intended applications include mobile TV, news, radio broadcasting, file delivery, emergency alerts, and others. When services are broadcasted by MBMS, all cells inside a multimedia broadcast multicast service single frequency network (MBSFN) area transmit the same MBMS service.
In order to acquire the amount of UEs which are using or interested in MBMS services, the base station is configured to initiate a counting procedure by sending a CountingRequest message. According to related 3GPP specifications, a UE can receive MBMS contents when operating in a radio resource control (RRC)-connected mode or an RRC-idle mode. However, only the UE operating in the RRC-connected mode is able to reply the CountingRequest message sent by the base station. In order for the UE operating in the RRC-idle mode to reply the CountingRequest message, the UE is required to perform a contention-based random access procedure for requesting an RRC connection. Therefore, in the prior art method of transmitting RRC messages when the UE operating in the RRC-idle mode only needs to send a single RRC message, many resources are required in order to establish an RRC connection between the base station and the UE, thereby influencing network efficiency.